1. Field of the Invention
The present invention generally relates to the field of vehicle transport systems, and more particularly to an overhead rail guided transport system having an automatic balancing function and an implementation method thereof.
2. Description of the Prior Art
With the continuous progress of the semiconductor industry, in the development and the design of ultra large scale integrated circuits (ULSI), the size of components have be reduced down to the nanometer degree in order to meet the requirements for designs of high density integrated circuits. Accordingly, there are hundreds of process steps for fabricating the required integrated circuits, such as etching, polishing, diffusion and deposition processes. That is to say, from the beginning to the end of the fabrication process, wafers in a same lot may be repeatedly transported from one to another tool for processing.
At present, in both interbay and intrabay regions, the wafers are transported by the overhead hoist transport system of an automated material handling system (AMHS). The overhead hoist transport system holds front open united pods (FOUPs) full of wafers, and transports the FOUPs among different processing tools along rails. However, there are still many drawbacks in the conventional transport system. For example, since each of the overhead hoist transport system is generally operated independently, the vehicles must be driven to certain maintenance areas and un-loaded by man power if they are to be changed to different overhead hoist transport systems. Also, due to the overhead hoist transport systems being independent, the vehicles can not be equilibrated automatically, which causes limited vehicle utilizing rates and wastes a lot of time during the exchanging steps.
Therefore, there is a need to provide a modified vehicle transport system with an automatic balancing function so that vehicles among different vehicle transport systems can be interchanged automatically. In this situation, the amount of empty vehicles may be reduced down to a certain degree and a nearly maximum vehicle utilizing rate may be obtained.